In recent years, requests for fuel-efficient vehicles have been more severe in association with global regulation of carbon dioxide emission in response to social needs of energy saving and growing interest in environmental issues. In order to meet such requests, tires exhibiting low rolling resistance are needed. Although optimization of the tire structure has been studied as a mean that decreases the rolling resistance of tires, the most typical means is to use materials exhibiting lower heat-buildup in rubber compositions.
High cis-1,4-polybutadiene that is prepared by polymerization using a catalyst containing a lanthanoid rare earth element is used as one of the rubber components in the rubber composition because it is a linear polymer generally having a small number of branched structures, has a higher cis content than conventional high-cis polybutadienes prepared by polymerization using a catalyst primarily containing cobalt, nickel, or titanium, and exhibits high abrasion resistance, heat-buildup resistance, and fatigue resistance.
Furthermore, studies to increase the cis content of the polybutadiene have been continuously conducted. For example, it is known that polymerization of butadiene using a catalyst system composed of a metallocene complex of a gadolinium compound gives a conjugated diene polymer having a significantly high cis-1,4 bond content. Unfortunately, since this polymer having a cis-1,4 bond content has a significantly narrow molecular weight distribution of 1.5 or less, rubber compositions containing this polymer exhibit poor workability and kneading characteristics, resulting in unsatisfactory mechanical properties (for example, refer to Nonpatent Document 1).
On the other hand, in order to formulate rubber compositions having low heat build-up, many techniques have been developed to enhance dispersion of fillers used in rubber compositions. Among them, the most typical method in a recent trend is modification of the polymerization active end of a diene polymer prepared by anionic polymerization using an organic lithium compound with a functional group that can interact with fillers.
For example, disclosed are a combination of use of carbon black as a filler and modification of polymerization active ends using a tin compound (for example, refer to Patent Document 1) and a combination of use of carbon black and introduction of amino groups into polymerization active ends (for example, refer to Patent Document 2).
Also, it is known that living polymers can be prepared by coordination polymerization using a catalyst containing a lanthanoid rare earth compound, and modification of the resulting polymers with specific coupling agents or modifiers are investigated (for example, refer to Patent Documents 3 to 5).
However, living ends generated using catalyst containing known lanthanoid rare earth compounds have low activity, that is, the terminal modification efficiency is several tens of percent at most, although techniques achieving a terminal modification efficiency of less than 75% have been disclosed recently (for example, refer to Patent Document 6). Although several examples of polymerization with high living characteristics have been reported, no example achieves compatibility of a high degree of microstructure control and a high conversion rate. These techniques can achieve the objects described above to some extent; however, a further improvement is awaited for achieving low fuel consumption required in the market.
[Patent Document 1] Japanese Examined Patent Application Publication No. 5-87530
[Patent Document 2] Japanese Unexamined Patent Application Publication No. 62-207342
[Patent Document 3] Japanese Unexamined Patent Application Publication No. 63-178102
[Patent Document 4] Japanese Unexamined Patent Application Publication No. 5-59103
[Patent Document 5] Japanese Unexamined Patent Application Publication No. 63-297403
[Patent Document 6] WO 95/04090
[Nonpatent Document 1] Macromol. Rapid Commun. 2003, Vol. 24, pp. 179-184